The present invention relates generally to gypsum compositions. More particularly, the invention relates to set gypsum-containing compositions that exhibit enhanced strength, dimensional stability, and resistance to permanent deformation, and to methods, gypsum-containing compositions, and products related thereto.
Set gypsum (calcium sulfate dihydrate) is a well-known material that is included commonly in many types of products. By way of example, set gypsum is a major component of end products created by the use of traditional plasters (e.g., plaster-surfaced internal building walls) and also in paper-faced gypsum boards employed in typical drywall construction of interior walls and ceilings of buildings. In addition, set gypsum is the major component of gypsum/cellulose fiber composite boards and products, and also is included in products that fill and smooth the joints between edges of gypsum boards. Also, many specialty materials, such as materials useful for modeling and mold-making that are precisely machined, produce products that contain major amounts of set gypsum.
Typically, such gypsum-containing products are prepared by forming a mixture of calcined gypsum (calcium sulfate hemihydrate and/or calcium sulfate anhydrite) and water (and other components, as appropriate). The mixture is cast into a desired shape or onto a surface, and then allowed to harden to form set (i.e., rehydrated) gypsum by reaction of the calcined gypsum with the water to form a matrix of crystalline hydrated gypsum (calcium sulfate dihydrate). It is the desired hydration of the calcined gypsum that enables the formation of an interlocking matrix of set gypsum crystals, thus imparting strength to the gypsum structure in the gypsum-containing product. Mild heating is utilized to drive off the remaining free (i.e., unreacted) water to yield a dry product.
One problem with such gypsum-containing products is that they often are subject to permanent deformation (e.g., sag), especially under conditions of high humidity, temperature, or load. For example, the possibility of sag is particularly problematic where gypsum-containing boards and tiles are stored or employed in a manner in which they are positioned horizontally. In this respect, if the set gypsum matrix in these products is not sufficiently resistant to permanent deformation, the products may start to sag in areas between the points to which they are fastened, or supported by, an underlying structure. This can be unsightly and can cause difficulties during use of the products. Furthermore, in many applications, gypsum-containing products must be able to carry loads, e.g., insulation or condensation loads, without perceptible sag.
Another problem with such set gypsum-containing products is that dimensional stability can be compromised during their manufacture, processing, and commercial application. For example, in the preparation of set gypsum products, there is usually a significant amount of free (i.e., unreacted) water left in the matrix after the gypsum has set. Upon drying of the set gypsum in order to drive off the excess water, the interlocking set gypsum crystals in the matrix tend to move closer together as the water evaporates. In this respect, as the water leaves the crystal interstices of the gypsum matrix, the matrix tends to shrink from natural forces of the set gypsum that were resisting capillary pressure applied by the water on the gypsum crystals. As the amount of water in the aqueous calcined gypsum mixture increases, lack of dimensional stability becomes more of a problem.
Dimensional stability is also of concern even after the final dried product is realized, especially under conditions of changing temperature and humidity where set gypsum is susceptible to, for example, expansion and shrinkage. For example, moisture taken up in crystal interstices of a gypsum matrix of a gypsum board or tile exposed to high humidity and temperature can aggravate a sagging problem by causing the humidified board to expand.
If such dimensional instability could be avoided or minimized, various benefits would result. For example, existing gypsum board production methods would yield more product if the boards did not shrink during drying, and gypsum-containing products desired to be relied upon to hold a precise shape and dimensional proportions (e.g., for use in modeling and mold making) would serve their purposes better.
International Publication No. WO 99/08979 describes the use of polyphosphates as an enhancing material to achieve the goals of resistance to permanent deformation (e.g., sag) and enhanced dimensional stability. According to WO 99/08979, polyphosphates in general produce the benefits of resistance to permanent deformation and enhanced dimensional stability whether the polyphosphates are used to treat a calcium sulfate material before it rehydrates to form set gypsum (i.e., a xe2x80x9cpre-set treatmentxe2x80x9d) or to treat an already-set gypsum (i.e., a xe2x80x9cpost-set treatmentxe2x80x9d). In many situations, carrying out a post-set treatment is more complicated and difficult to achieve than a pre-set treatment because, for example, once the set gypsum is in place, it can be difficult to absorb the enhancing material into the set gypsum or to contact the set gypsum with the enhancing material (e.g., where the set gypsum is covered with paper). Thus, it generally is more desirable to impart resistance to permanent deformation and enhanced dimensional stability by way of a pre-set treatment of a calcium sulfate material. However, almost all of the polyphosphates described in WO 99/08979 have a negative effect on the strength of the resulting set gypsum-containing product when used in a pre-set treatment.
Thus, there is a continuing need to impart resistance to permanent deformation and enhanced dimensional stability in a pre-set treatment while maintaining or, preferably, even increasing the strength (e.g., compressive strength) of the resulting set gypsum-containing product. Of the polyphosphate enhancing materials disclosed in WO 99/08979, only the trimetaphosphate compounds were found to actually increase strength upon addition in a pre-set treatment. It would be desirable to identify alternative enhancing materials for achieving resistance to permanent deformation and enhanced dimensional stability in a pre-set treatment of a calcium sulfate material while maintaining or, preferably, even increasing the strength of the resultant set gypsum product so as to afford flexibility in manufacture and in choosing raw materials.
Accordingly, it will be appreciated from the foregoing that there is a need in the art for an alternative pre-set treatment of a calcium sulfate material for increasing resistance to permanent deformation (e.g., sag) and dimensional stability while maintaining or, preferably, even increasing the strength of the resultant set gypsum product. The invention satisfies this need. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
The present invention provides a composition comprising a calcium sulfate material, a set gypsum-containing product, and a method for producing a set gypsum-containing product having increased resistance to permanent deformation, while maintaining or increasing strength.
In accordance with the present invention, it has been found that the inclusion of a tetrametaphosphate compound (e.g., salt or ion) in a pre-set treatment of a calcium sulfate material enhances the resistance to permanent deformation (e.g., sag) while maintaining or, preferably, increasing compressive strength of the composition. Furthermore, it is expected that the addition of the tetrametaphosphate in a pre-set treatment will enhance the dimensional stability of the resulting set gypsum-containing product. It is to be noted that, although the singular is utilized for convenience to describe certain terms such as the tetrametaphosphate compound and the calcium sulfate material, it will be understood that singular terms such as xe2x80x9ca,xe2x80x9d xe2x80x9can,xe2x80x9d and xe2x80x9cthexe2x80x9d are meant to cover both the singular and the plural (e.g., tetrametaphosphate compounds, calcium sulfate materials, or the like), unless otherwise indicated herein or clearly contradicted by context.
Thus, in one aspect, the present invention provides a composition comprising a mixture of a calcium sulfate material, water, and a tetrametaphosphate compound. As used herein, the term xe2x80x9ccalcium sulfate materialxe2x80x9d refers to calcium sulfate anhydrite, calcium sulfate hemihydrate, ions of calcium and sulfate, or mixtures of any of the foregoing, but is not meant to refer to calcium sulfate dihydrate (i.e., set gypsum). In some embodiments, an accelerator for increasing the rate of the hydration of the calcium sulfate material to produce calcium sulfate dihydrate (i.e., set gypsum) optionally is included in the composition, for example, when the composition is used to prepare a board. Some embodiments optionally also can include pregelatinized starch, aqueous foam, and/or other ingredients as will be appreciated readily by one of ordinary skill in the art.
The composition of the invention is useful to produce set gypsum-containing products that exhibit sag resistance while maintaining or increasing compressive strength. Desirably, when the composition is cast to form a xc2xd inch board comprising an interlocking matrix of set gypsum, the board has a sag resistance, as determined according to ASTM C473-95, of less than about 0.1 inch per two-foot length of said board. Also, when the composition is used to prepare a product comprising an interlocking matrix of set gypsum, the product has at least as much strength, and preferably has more strength, than it would have if the tetrametaphosphate compound had not been included in the mixture.
In some embodiments, the composition comprises set gypsum and host particles such that at least a portion of the set gypsum is positioned in and about accessible voids in the host particles. As a result, in another aspect, the present invention provides a composition comprising a mixture of the host particles having the accessible voids therein; calcium sulfate hemihydrate, at least a portion of which is in the form of crystals in and about the voids of the host particles; and a tetrametaphosphate compound. When the composition is cast to form a xc2xd inch board comprising an interlocking matrix of set gypsum, the board preferably has a sag resistance, as determined according to ASTM C473-95, of less than about 0.1 inch per two-foot length of the board, and the board has at least as much strength, and preferably has more strength, than it would have if the tetrametaphosphate compound had not been included in the mixture.
In some embodiments of the invention, the composition is mixed with water to produce a machinable set gypsum-containing product or a joint compound. The machinable set gypsum-containing product comprises a mixture of a starch, particles of a water-redispersible polymer, calcium sulfate hemihydrate, and a tetrametaphosphate compound. The joint compound is useful for finishing joints between edges of gypsum boards, and comprises a mixture of a binder, a thickener, a non-leveling agent, calcium sulfate hemihydrate, and a tetrametaphosphate compound.
Some embodiments of the inventive composition are useful for producing an acoustical tile. Such compositions comprise a mixture of a calcium sulfate material, water, and a tetrametaphosphate compound, as well as one or more of a gelatinized starch, a mineral wool, expanded perlite particles, and/or a fiber reinforcing agent. By way of example, in some embodiments, the composition comprises a mixture of a gelatinized starch, a mineral wool, a calcium sulfate material, water, and a tetrametaphosphate compound. As another example, in some embodiments, the composition comprises a mixture of a gelatinized starch, expanded perlite particles, a fiber reinforcing agent, a calcium sulfate material, water, and a tetrametaphosphate compound.
In another aspect of the invention, a method for producing a set gypsum-containing product having increased resistance to permanent deformation is provided. The method comprises forming a mixture of a calcium sulfate material, water, and a tetrametaphosphate compound. The mixture is maintained under conditions sufficient for the calcium sulfate material to form an interlocking matrix of set gypsum material. The tetrametaphosphate compound is included in the mixture in an amount such that the set gypsum-containing product has greater resistance to permanent deformation than it would have if the tetrametaphosphate compound had not been included in the mixture.
In yet another aspect of the invention, a method for producing a set gypsum-containing product having enhanced strength is provided. The method comprises forming a mixture of a calcium sulfate material, water, and a tetrametaphosphate compound. The mixture is maintained under conditions sufficient for the calcium sulfate material to form an interlocking matrix of set gypsum material. The tetrametaphosphate compound is included in the mixture in an amount such that the set gypsum-containing product has greater strength than it would have if the tetrametaphosphate compound had not been included in the mixture.
In still another aspect, the present invention provides a set gypsum product. In some embodiments, the set gypsum product is a gypsum board. For example, the present invention provides gypsum board comprising an interlocking matrix of set gypsum formed from, i.e., using, at least calcined gypsum, water and a tetrametaphosphate compound. Preferably, the gypsum board has a sag resistance, as determined according to ASTM C473-95, of less than about 0.1 inch per two-foot length of the board, while maintaining or increasing strength.
In some embodiments, the set gypsum product is a gypsum board comprising a core of material sandwiched between cover sheets. The core comprises an interlocking matrix of set gypsum. Preferably, the board is prepared by a method comprising forming or depositing a mixture between the cover sheets, wherein the mixture comprises a calcium sulfate material, water, and a tetrametaphosphate compound. The mixture is maintained under conditions sufficient for the calcium sulfate material to form the interlocking matrix of set gypsum. The tetrametaphosphate compound is included in the mixture in an amount such that the gypsum board has a sag resistance, as determined according to ASTM C473-95, of less than about 0.1 inch per two foot length of the board, while maintaining or increasing strength.
The invention may best be understood with reference to the accompanying drawings and in the following detailed description of the preferred embodiments.